Fiber composite materials have been used in a wide range of industries (e.g., aerospace, aviation, automobile, ship, construction, sports equipments, etc.) for several decades. Fiber composite materials are generally manufactured through a process where fiber products are laid up filament by filament, ply by ply, and layer by layer, then bonded together using materials, such as plastics, rubber, metal, or other non-metal materials. A connection interface exists between two filaments/plies/layers. Fiber composite materials are usually weak at connection interfaces or inter-laminate area. For example, tensile strength of fibers within a ply can be higher than 3000 MPa (Mega Pascal), much higher than that of common steel; but interface or inter-laminate strength of fiber composite materials is usually about 100 MPa, even lower than that of Aluminum. Thus, the interface or inter-laminate strength of fiber composite materials is only about 1/30 of the tensile strength of fiber themselves.
It is generally difficult and expensive to increase the interface or inter-laminate strength of the filer composite materials by using a stronger bonding material (e.g., adhesives, plastics, or resin, etc.). The lower interface or inter-laminate interface strength of composites often leads to structural failures, such as de-lamination and de-bonding, under conditions, such as stress, impact, or fatigue. Therefore, adoption of fiber composite materials, especially in load-bearing structures, has been limited by their susceptibility to out-of-plane failures caused by low interface or inter-laminate strength. To alleviate these problems, methods of trans-laminate and cross-interface reinforcements, such as 3-D stitching, 3-D weaving, 3-D knitting, or 3-D braiding, have been used to improve de-lamination resistance and enhance interface strength. These 3-D reinforcement techniques, however, demand sophisticated equipments and require complicated manufacturing processes, and therefore, prolong the manufacture process, drive up the manufacture cost, and lower the manufacture productivity.